Device and method for thermogravimetrically testing the behavior of a solid material

ABSTRACT

A method for thermogravimetrically testing the behavior of a solid material in the presence of a controlled gaseous atmosphere, characterized in that a plurality of samples ( 10 ) are placed in the presence of the gaseous atmosphere inside the same controlled atmosphere furnace ( 4 ); each sample is associated with a scale ( 38 ) proper thereto; the samples ( 10 ) undergo predetermined successive thermal cycles each including a heating step during which the samples are directly heated (by radiation or induction) and a cooling step during which the weight of each sample is independently measured and recorded in a continuous manner during at least one predetermined period such as a high temperature level during the heating step of each thermal cycle. The invention also relates to a device for carrying out the method.

FIELD OF THE INVENTION

The invention relates to a device and method for thermogravimetricallytesting the behaviour of a solid material subjected, in the presence ofa gaseous atmosphere, to considerable variations in temperature.

BACKGROUND OF THE INVENTION

The majority of materials (metal alloys, ceramics, concretes, etc.)oxidise under the effect of an elevated temperature in the presence of agaseous atmosphere. This phenomenon of oxidation is amplified andbecomes damaging when the material is subjected to considerable cyclicvariations in temperature: during the first cycles, exposure of thematerial to high temperatures causes the formation of a protective oxidelayer (transitory oxidation phase), which grows until it reaches acritical thickness above which any cooling to which the material issubjected causes flaking of the protective oxide layer; duringsubsequent cycles, cooling operations to which the material is subjectedcause the protective oxide layer to break up, and successive oxidationsat high temperature of the surface of the material thus exposed depleteit of elements permitting the formation of the protective oxide, toexhaustion; cyclic exposure of the material to high temperatures thenmanifests itself as in-depth oxidation of the material (formation ofmetal oxide sub-layers, which flake during the cooling phases), whichthus attacks the material until it breaks.

In addition to oxidation of the material, exposure of a material to hightemperatures can also cause phenomena of corrosion, passivation oradsorption, which manifest themselves as a gain in mass of the material,or phenomena of decomposition, dehydration, pyrolysis, combustion ordehydroxylation, which manifest themselves as a loss in mass of thematerial. These phenomena can be observed and studied bythermogravimetry.

The thermogravimetric study of the behaviour of a material subjected, inthe presence of a gaseous atmosphere, to considerable variations intemperature is fundamental to many industries: aviation (gas turbines,especially of aircraft engines), the automotive industry (exhaust pipes,catalytic converters, etc.), chemical engineering (chemical andpetrochemical factory reactors), the nuclear industry, the electricalindustry (thermal generators), etc. It allows evaluation of, inter alia,the resistance of a material to thermochemical attacks, its lifetime,the risks of cracking, the maximum possible use temperature of thematerial, etc., and makes it possible to work at developing new,higher-performance materials which are capable of withstanding higheruse temperatures. The thermogravimetric study of the behaviour of amaterial at high temperature is also of interest ecologically:increasing the use temperatures of materials results in a greaterefficiency of the industrial processes in which the materials areinvolved, and a subsequent reduction in energy consumption, CO₂emissions, etc.

During use, materials undergo attacks of various origins: attacks ofthermal and/or chemical origin (high-temperature oxidation, cyclicoxidation, corrosion, decomposition, dehydration, etc.), mechanicalstresses, cyclic thermo-mechanical fatigue, etc. These various factorsinteract in a complex manner. And there is at present no laboratory testthat is capable of reproducing all these factors at reasonable cost andin reduced times.

For a material subjected to elevated temperatures (from 400 to 1800° C.)and/or to considerable variations in temperature, thermal phenomena (andespecially the phenomena of oxidation and/or corrosion at hightemperature, depending on the composition of the gaseous atmosphere)often prove to be the most important. For that reason, the lifetime of amaterial subjected to such use conditions is evaluated on the one handby means of simplified and “accelerated” thermogravimetric tests, whichallow measurement of the effects of exposure (isothermal or cyclic) ofthe material to high temperatures for a given period of time, and on theother hand by means of mathematical simulation models which, whenapplied to the experimental results of the preceding thermogravimetrictests, make it possible to simulate and extrapolate not only thelong-term action of cyclic high-temperature exposure (as a function ofthe actual use conditions of the material—chemical composition of theatmosphere, maximum temperature of the material, time of exposure tohigh temperature for each cycle, number of cycles, rate of heating, rateof cooling, etc.—which are often different from the test conditions),but also the effects of other possible factors (mechanical,thermo-mechanical, etc.), the interaction of the various factors, therandom nature of certain phenomena, a statistical modulation of theexperimental results.

THE PRIOR ART

Two types of thermogravimetric test are known at present:

-   -   isothermal thermogravimetric tests, in which a sample of        material is placed in a furnace and exposed to an elevated        temperature corresponding to a potential use temperature of the        material, and the gain in mass (or in weight) of the material        following this exposure is measured (the term “gain” denoting a        relative gain: it is positive in the case of an increase in mass        and negative in the case of a loss of mass). In the case of an        oxidation test, measurement of the gain in mass of the sample        provides information regarding the amount of oxide formed. In        all cases, measurement of the gain in mass of the sample is        carried out either by means of a “conventional” balance at        ambient temperature (the sample is in this case removed from the        furnace beforehand and placed on the balance, and the test is        described as discontinuous), or by means of a thermobalance such        as a SETARAM® electric resistance thermobalance (the resistance        furnace and the balance being combined in a single device) while        the sample is still in the furnace (such a test is described as        continuous). Continuous isothermal tests are more reliable than        discontinuous tests: the handling, cooling and change in        hygrometry which the sample undergoes within the scope of a        discontinuous test, during its transfer from the furnace to the        balance, cause an effect of premature flaking of the oxide layer        and losses of oxide particles, as well as charging of the sample        with moisture, which falsify the mass-gain measurements;    -   cyclic thermogravimetric tests, in which a sample is subjected        to cyclic variations in temperature, each cycle comprising a        heating step in a furnace and a cooling step outside the        furnace, and the weight of the sample is measured periodically,        between two cycles. These tests are carried out by means of a        combustion or electric resistance furnace. Each cooling step is        carried out at ambient temperature, outside the furnace, from        which the sample is removed manually or automatically. The        sample is weighed regularly outside the furnace, after a given        number of cycles (by way of example, the sample is subjected to        successive 2-hour cycles and is measured once per day, that is        to say approximately every 11 cycles).

The major disadvantage of isothermal tests is that they do not reproducerealistic use conditions for industrial materials. In fact, it is rarethat materials are subjected, during use, to constant temperaturesthroughout their lifetime. In the large majority of cases, the materialsare subjected to cyclic variations in temperature (owing to thediscontinuous use of industrial installations). However, the effect,mentioned hereinabove, of cyclic cooling on the durability of thematerial is fundamental. Interpreting the results of isothermal testswith a view to predicting the effects of cyclic oxidation/corrosion (orother attack) is difficult. The value and reliability of isothermaltests are consequently relatively limited.

By contrast, cyclic tests make it possible to take into considerationthe effects of cooling to which the material is subjected in the processof oxidation/corrosion (or other phenomenon). However, the known cyclictests do not allow the re-creation of test conditions which are similar,or at least are capable of permitting reliable extrapolation, to theactual use conditions of the material being tested. Moreover, the knowncyclic tests are discontinuous tests (weight measurements are carriedout outside the furnace) and as such have the same disadvantages as thediscontinuous isothermal tests. These tests must therefore beinterpreted with caution. The results that they provide are, moreover,insufficient to permit fine analysis of the phenomena in question.Moreover, the tests take a long time to carry out and require thefrequent intervention of a technician (especially to carry out theoperations of weighing the samples of material).

In addition, the known isothermal tests, when they are discontinuous,only provide the net gain in mass (gain in mass of the sample alone)following the exposure to high temperature. Whatever the final purposeof the test (study of oxidation, dehydration, etc.), this parameter isinsufficient for permitting analysis of the phenomena in question andevaluation of the lifetime of the material. In the case of an oxidationtest in particular, measurement of only the net gain in mass followingthe exposure to high temperature does not allow the increase in mass ofthe sample due to adsorption of oxygen (formation of the oxide) to beseparated from the loss in mass due to flaking of the oxide. Theinformation provided by the test therefore remains inadequate.

Likewise, the known cyclic tests, which are discontinuous, only provideeither the net gain in mass between two cycles when the sample issuspended in the furnace by means of a hook, or both the net gain inmass and the gross gain in mass (variation in mass both of the sampleand of the oxide lost by flaking) between two cycles when the sample isplaced in a crucible inside the furnace. The disadvantages of the firstmethod have been discussed hereinabove. The second method providesimportant additional information but raises a number of experimentalproblems: it is impossible to obtain rates of heating/cooling of thesample that are elevated—and a fortiori realistic—inside a crucible; thepresence of the crucible alters the gaseous environment (which cannot behomogeneous) of the sample; the variation in mass of the crucible itselfreduces the accuracy of the measurements; the handling of the samplethat is necessary to measure its net gain in mass (removal from thecrucible) risk damaging the layer of oxide and falsifying themeasurements. These comments are also applicable to any test other thanan oxidation test.

It is to be noted that there also exist known methods for characterisinga material (determining its chemical composition—nature and proportionof its constituents, its degree of polymerisation and/or crosslinking ifit is a polymer, etc.) which use thermogravimetric techniques to detectthe changes undergone by the material (revealed by the variations inweight of the sample) when it is subjected to an increase intemperature. By determining the temperatures at which such changes occurand the quantity of material lost or gained by the sample, it ispossible to determine the nature of its constituents.

It is not the aim of these known characterisation methods, nor do theymake it possible, to predict the behaviour of a material subjected,during use, to an aggressive gaseous atmosphere and to pronouncedvariations in temperature. Furthermore, none of these methods providesfor the material to be subjected to predefined temperature cycles inorder to reproduce or model the actual use conditions of the material,with a view to studying the phenomena that occur during oxidation and/orcorrosion and/or dehydration, etc. of the material and to predict itslifetime. And the devices they use are unsuitable for this type of test.

Nevertheless, mention may be made, by way of information, of U.S. Pat.No. 5,638,391, which describes a characterisation method in which therate of heating of the sample is constantly regulated (automaticcontrol) as a function of the measured weight, in order to impose a verylow rate (less than 10° C./minute) during changes and a higher rate(from 50 to 100° C./minute) outside changes, with a view to achievingbetter resolution (separation of the possible changes occurring atsimilar temperatures) and a reduction in the duration of the test. Thethermal test conditions (temperatures, rates of heating/cooling,duration, etc.) are therefore defined as the test progresses, accordingto the weight of the sample. Real use conditions of the material are notsimulated at all. WO 01/34290 describes a device for synthesising andcharacterising an individual chemical (polymer) and/or biologicalcompound from a combinatory library, which comprises rows of synthesiscrucibles arranged in parallel lines. Each crucible is subjected to arise in temperature, obtained by indirect heating means such as heatingchannels running between the crucibles, while an alternating voltage isapplied to an electrode which extends beneath a flexible membrane havinga reflective face, which forms the base of the crucible. The resultingalternating electric field causes the membrane to vibrate, the amplitudeof the vibration, measured by an optical sensor beneath the membrane,depending on the mass of polymer contained in the crucible. EP 779 510describes an apparatus for analysing the composition of a gas,comprising a sensor or a row of sensors. Each sensor is composed of anpiezoelectric crystal anode enclosed in a casing for receiving a volumeof gas, which anode is covered with a metal capable of adsorbing theparticles of a given component (according to the nature of the metalcoating) of the gas by electrostatic precipitation, then releasing theadsorbed particles by oxidation under the effect of heat. Thepiezoelectric anode allows the variations in weight of the metal coatingto be determined, by measurement of its oscillation frequency, when itadsorbs or releases particles of gas. Such an apparatus is designedspecifically for testing gases and is not suitable for testing thebehaviour of solid materials, in particular because it does not havemeans for receiving and measuring the weight of a sample of solidmaterial.

OBJECT OF THE INVENTION

The invention aims to remedy these disadvantages by proposingprincipally a method for thermogravimetrically testing the behaviour ofa material when it is subjected to pronounced variations in temperaturein the presence of a controlled gaseous atmosphere, which method is morereliable, more accurate and more rapid than the known test methods. Theinvention aims also to provide a device for carrying out such a method.

The object of the invention is especially to provide a method and adevice allowing the material to be tested under thermal conditions(temperature of the material, rates of heating and cooling, duration ofthe thermal cycles, etc.) which are suitable for permitting a reliableprediction of the behaviour of the material under its actual useconditions.

In particular, it is an object of the invention to propose a method inwhich the material is tested under thermal conditions that are verysimilar to its actual use conditions, and a device capable ofreproducing such thermal conditions. Another object of the invention isto propose a method in which the material is tested under so-calledenvironmental conditions (pressure and chemical composition of theatmosphere in which it is carried out) which are similar to its actualuse conditions, and a device capable of reproducing such environmentalconditions.

Another object of the invention is to provide a method and a devicewhich make it possible to evaluate and accurately monitor, in eachthermal cycle, the evolution of the behaviour of the material and,especially, in the case of an oxidation test, the amount of oxideformed, the oxidation kinetics, the amount of oxide lost by flaking, theflaking kinetics, the average thickness of the remaining layer of oxide,its adherence to the metal, etc, without these evaluations requiringadditional handling of the material or prolonging the duration of thetest.

Another object is to provide a method and a device which make itpossible to evaluate with greater reliability the statistical behaviourof a material subjected to considerable variations in temperature in thepresence of a given gaseous atmosphere. In particular, the inventionaims to provide a device which allows the same experiment to bereproduced several times under identical environmental and thermalconditions.

Another object of the invention is to provide a compact test device.

The invention also aims to reduce considerably the cost and the durationof a test without prejudicing the accuracy, reliability and pertinenceof the test.

SUMMARY OF THE INVENTION

The invention relates to a method for thermogravimetrically testing thebehaviour of a solid material in the presence of a controlled gaseousatmosphere, wherein:

-   -   a plurality of samples are placed in the presence of said        gaseous atmosphere inside the same controlled-atmosphere        furnace,    -   each sample has its own associated balance having an error of        less than 100 μg; it is to be noted that the term “balance” is        understood as meaning any means of measuring weight or mass or        of measuring the variation in weight or mass, without any        exclusion regarding the type of balance (so-called beam balance,        Roberval balance, electronic weighing cell,        displacement-measuring optoelectronic sensor, voltage-measuring        electronic sensor, etc.),    -   the samples are subjected to predetermined successive thermal        cycles each comprising a heating step, during which the samples        are heated directly, and a cooling step, during which the        samples are not heated; it is to be noted that the expression        “the samples are heated directly” means that the samples are        heated in a direct manner by radiation or induction, for        example, in contrast to indirect heating methods (using heating        means of the thermal electric resistance, combustion gas type,        etc.), which consist in heating the atmosphere surrounding the        samples in order to increase their temperature,    -   the weight of each sample is measured and recorded        independently, in a continuous manner, for at least a        predetermined period during the heating step of each thermal        cycle. It is to be noted that the measured weight corresponds to        the absolute weight of the sample if the balance is adjusted to        zero when empty, or—preferably—to a relative weight of the        sample relative to its initial weight if the balance is adjusted        to zero at the start of the test, provided with the sample.

In particular, the weight of each sample is measured and recorded in acontinuous manner at least during a high-temperature stage of theheating step of each thermal cycle. It is to be noted that it is equallypossible to measure and record the weight of each sample during a periodof the cooling step, or even throughout the entirety of the test.However, in the particular case of an oxidation test, the inventors haveshown that a study of the variations in weight (or in mass) of eachsample during a high-temperature stage of the heating step is sufficienton its own for describing the oxidation phenomena sensitively andreliably, as will be explained hereinbelow.

According to the invention, therefore, and contrary to the known cyclictests (which provide phases of cooling of the sample in the open air,outside the furnace and away from any controlled atmosphere), thesamples are placed in a controlled-atmosphere furnace in order to besubjected to thermal cycles and are not removed therefrom until the endof the cyclic test. They are therefore exposed, permanently (during eachcycle and between cycles), for the entire duration of the test, to acontrolled gaseous atmosphere. The samples can thus be maintained andobserved in an environment (pressure and chemical composition of theatmosphere surrounding them) that has been created so as to correspond,as closely as possible, to their actual use environment.

In addition, contrary to the known cyclic tests, the weighing operationsare carried out without handling of the samples by a technician or evenby the device. The samples are not subjected to any mechanical stresswhich might modify their mass in an undesirable manner and in particulardamage the oxide formed and facilitate flaking thereof. Consequently, itis possible accurately to observe the effects of only the thermal cycleson the material; and the results obtained are particularly reliable.

Moreover, the weighing operations are carried out automatically duringthe thermal cycles (and not between two thermal cycles), withoutinterrupting said cycles. In addition to the possibility of carrying outsuccessive thermal cycles in a continuous manner (like the actual cyclesto which the material is subjected in use), an appreciable andsignificant gain in terms of time is achieved.

Furthermore, according to the invention and contrary to all the knowncyclic tests, which provide the gain in mass (net or gross) in a limitedmanner between two cycles, the weight of each sample present in thefurnace is measured and recorded continuously at least over a givenperiod during the heating step, and especially during a high-temperaturestage of that step. From these measurements it is possible to deduce thevalues of a large number of rich information parameters, some of whichwere previously inaccessible, such as—in the case of an oxidationtest—the exact amount of oxide formed in each cycle on each sample(given by the gain in mass of the sample during the high-temperaturestage), the kinetics of formation of the oxide (and the inventors haveshown that this gives information regarding the nature of the oxideformed), the exact amount of oxide lost by flaking in each cycle by thesample (given by the difference in weight of the sample between the endof a high-temperature stage and the start of the followinghigh-temperature stage), the thickness of the layer of oxide formed(obtained by calculation), etc. And this information is obtained withouthandling the sample and solely by weighing the sample alone (and not theoxide lost by flaking) for a predetermined period during the heatingstep (chosen according to the phenomena to be studied and according tothe material). In the case of an oxidation test, this periodadvantageously corresponds to a high-temperature stage, the inventorshaving shown that the formation of the oxide on the material occurssubstantially during such a stage. The choice of this period has anotheradvantage: at a substantially constant temperature, the thermal currentsand the effect of the variations in the buoyancy to which the samplesare subjected are negligible; the weight measurements are therefore morereliable. The exploitation of the weight measurement results of thesamples outside such stages is more delicate and proves to be of littleuse.

The method according to the invention accordingly permits finer and morereliable analysis of cyclic oxidation/corrosion phenomena at hightemperature. In particular, it allows the evolution of the behaviour ofthe material to be observed not only during one thermal cycle but alsoand especially from one cycle to another, at various stages of theoxidation method, and allows the lifetime of the material to bepredicted in an accurate and reliable manner. These comments areapplicable whatever the phenomenon studied (decomposition, pyrolysis,dehydration, etc.).

In addition, contrary to all the known tests using thermobalances(isothermal tests), the method according to the invention consists notonly in subjecting the material to thermal cycles, but also in weighing,simultaneously and independently, a plurality of samples placed in thesame gaseous atmosphere, that is to say placed under strictly identicalenvironmental conditions. The samples are weighed by means ofindependent balances which are able to operate concomitantly, eachbalance measuring the weight of a single sample with an error of lessthan 100 μg. By means of the method according to the invention it istherefore possible to reproduce the same experiment several times underidentical conditions, in a limited time, at reduced cost and usingreduced means but with a high degree of accuracy. Consequently, it ispossible to carry out reliable statistical studies on the measurementresults obtained.

It is to be noted, moreover, that the known thermobalances, which aredesigned exclusively for carrying out isothermal tests, do not allowexploitable cyclic tests to be carried out. In fact, the rates ofheating and cooling which can be obtained inside the furnace of a knownthermobalance are low (of the order of 60° C./minute for heating and 30°C./minute for cooling) and, for the large majority of materials, are outof proportion relative to the actual rates of heating and cooling of thematerial during use (in particular in the case of an aviation material).This is one of the reasons why the known thermobalances are unsuitablefor cyclic tests.

The invention also consists in using means for heating the samplesdirectly. Such means offer rates of heating and cooling of the samplesthat are superior to the indirect heating means used in the methods ofthe prior art, and allow thermal cycles comprising very short phases ofrise and fall in temperature to be carried out. The method and thedevice according to the invention are suitable for the most demandingapplications, such as aviation applications.

Advantageously and according to the invention, one or more of thefollowing operations are carried out:

-   -   in each thermal cycle, the samples are heated so that their        temperature is from 400 to 1800° C. at least during a        high-temperature stage of the heating step, and especially        greater than 1100° C. at least during such a stage;    -   in each thermal cycle, the samples are heated at a rate of        heating greater than 300° C./minute (in other words, the samples        are heated in such a manner as to increase their temperature by        more than 300° C. in one minute), or even greater than 1000°        C./minute;    -   in each thermal cycle, the samples are cooled at a rate of        cooling greater than 100° C./minute (in other words, the samples        are cooled in such a manner as to lower their temperature by        more than 100° C. in one minute);    -   according to the nature and intended use of the material being        tested, the samples are subjected to a number of successive        thermal cycles of from 10 to 3000;    -   the samples are subjected to thermal cycles each comprising a        heating step, which consists of a phase of rise in temperature        having a duration of less than 5 minutes and a high-temperature        stage having a duration of the order of 60 minutes, and a        cooling stage, which consists of a phase of fall in temperature        having a duration of less than 10 minutes and a low-temperature        phase having a duration of from 0 to 15 minutes.

By way of example, in the case of a tested material of the superalloytype intended for aviation applications (turbines, nozzles, etc.), thesamples are subjected to a number of thermal cycles varying from 300 to3000, each cycle comprising a phase of rise in temperature of at least 2minutes, a high-temperature stage of approximately 60 minutes, duringwhich the samples are maintained at a temperature of from 1100° C. to1500° C., and a phase of fall in temperature of approximately 4 minutes,which allows the samples to be returned to a temperature of from 100 to200° C., and a low-temperature stage (from 100 to 200° C.) for a periodof from 0 to 15 minutes.

In the case of a ceramics-type material, the samples are brought to1800° C. within a period of less than 3 minutes (phase of rise intemperature) and maintained at that temperature during thehigh-temperature stage.

In the case of a material of the steel or alloy type intended forautomotive pipework applications (exhaust pipe) or for the chemicalindustry, the temperature of the high-temperature stage is of the orderof 500° C., the temperature of the low-temperature stage is from 10 to30° C. and the duration of the high- and low-temperature stages mayvary, according to the application, from several minutes to severalhours.

It is to be noted that the thermal cycles of the same cyclic test may beidentical or different. In particular, they may have different maximumtemperatures and/or different minimum temperatures and/or differentdurations of the heating step or the high-temperature stage and/ordifferent durations of the cooling step or the low-temperature stageand/or different rates of heating and/or different rates of cooling,etc.

The invention relates also to a device for thermogravimetrically testingthe behaviour of a solid material in the presence of a controlledgaseous atmosphere, which device enables the method according to theinvention to be carried out. The invention relates especially to adevice comprising:

-   -   a furnace with a controlled gaseous atmosphere,    -   means for weighing the material placed in the furnace, having an        error of less than 100 μg,    -   confining means suitable for limiting any disturbance to the        weighing means owing to the external environment of the device        and/or the controlled gaseous atmosphere of the furnace.

It is characterised in that:

-   -   the furnace is suitable for receiving a number N, which is        strictly greater than 1, of samples of the material (that is to        say a plurality of samples),    -   the furnace comprises means for heating the samples directly,        which means are capable of subjecting the samples to successive        predetermined thermal cycles each comprising a heating step,        during which the samples are heated, and a cooling step, during        which the samples are not heated,    -   the weighing means comprise N independent balances having an        error of less than 100 μg, each balance being capable of        measuring and recording the weight of a sample continuously at        least during a predetermined period during the heating step of        each thermal cycle. In particular, each balance is capable of        measuring and recording the weight of a sample continuously at        least during a high-temperature stage of the heating step of        each thermal cycle.

The expression “direct heating means” denotes means of the radiant orinductive type which are capable of heating the samples of materialdirectly without necessarily heating the atmosphere surrounding them.

The device is preferably star-shaped in its overall structure, at leastthe balances being arranged in the shape of a star. Such a structure isparticularly compact.

Advantageously and according to the invention, this star-shapedstructure is suitable for receiving the samples close to one another ina central portion of the furnace. Such a structure allows the samples tobe arranged in the central portion of the furnace within a limitedvolume, the form and dimensions of which facilitate the creation of ahomogeneous atmosphere. The samples are therefore subjected strictly tothe same atmosphere at all times.

In addition, because the receiving volume for the samples is limited, itis possible, by closing that volume at least partially, to limit theeffect on the weighing operation of the gaseous currents due to thermalvariations (thermal currents), to a circulation of gas with a view tomaintaining or voluntarily changing the controlled atmosphere (supply tothe furnace or extraction), until that influence is rendered negligible.This would not be the case in a device in which the samples and theirassociated balances were aligned in rows, and in which it would beadvisable to take those effects into account when studying the behaviourof the material.

Advantageously and according to the invention, the direct heating meansare capable of bringing the samples to a temperature greater than 400°C., and especially greater than 1100° C., or even greater than 1800° C.,of heating the samples at a rate of heating greater than 300° C./minute,or even greater than 1000° C./minute, and of cooling the samples at arate of cooling greater than 100° C./minute. The direct heating meansare preferably capable of carrying out thermal cycles each comprising aheating step, which consists of a phase of rise in temperature having aduration of less than 5 minutes and a high-temperature stage having aduration of the order of 60 minutes, and a cooling step, which consistsof a phase of fall in temperature having a duration of less than 10minutes and a low-temperature stage having a duration of from 0 to 15minutes. The direct heating means are preferably capable of carrying outmore than 3000 successive thermal cycles.

In a preferred embodiment of the invention, the furnace comprises:

-   -   at least N high-radiation lamps, such as halogen lamps,    -   a receiving chamber for the samples made of a thermal resistant        material which is transparent to the radiation of the lamps        (visible and/or infra-red and/or ultraviolet radiation,        according to the nature of said lamps); in particular, the        chamber is made of optical-grade quartz (such a material is        transparent to visible radiation and is heated only slightly        under the effect of such radiation); it is to be noted that the        term “chamber” denotes both the confined space for receiving the        samples inside the furnace and the wall (of quartz) delimiting        that space,    -   a reflective peripheral inner face having a form suitable for        defining at least N separate zones of maximum illumination        inside the chamber, at the site of which the samples may be        placed; the expression “zone of maximum illumination” denotes a        convergence zone of the radiation emitted by the lamps and        reflected by the inner peripheral face of the furnace.

Advantageously and according to the invention, the peripheral inner faceof the furnace forms at least N ellipse portions arranged in the shapeof a star, each ellipse having a first focus outside the chamber, calledthe emitting focus, at the site of which there is arranged a lamp, and asecond focus inside the chamber, called the receiving focus, at the siteof which a sample of the material may be placed. According to theinvention, at least N ellipses have separate receiving focuses. Thechamber and the receiving focuses are preferably situated in the centralportion of the furnace, and the emitting focuses are situated in theperipheral portion of the furnace. The chamber advantageously hasreduced radial dimensions, preferably just sufficient to house thesamples.

Such heating means have an advantageous flexibility, especially rates ofheating and cooling which can be altered as desired, the rates ofheating readily being able to exceed 300° C./minute (and even 1000°C./minute) and the rates of cooling being able to exceed 100° C./minute.They also allow the samples to be brought to very high temperatures (ofthe order of 1800° C.). They therefore provide the possibility ofcarrying out different thermal cycles as a function of the nature andintended use of the material being tested, and of reproducing, in thelarge majority of cases, thermal conditions similar to the actual useconditions of the material.

Advantageously and according to the invention, each balance has an errorof less than 10 μg and especially of the order of 1 μg. Advantageouslyand according to the invention, each balance has a drift of less than 10μg/h, preferably less than 1 μg/h and especially of the order of 0.1μg/h.

Advantageously and according to the invention, the balances are mountedon the same support plate. They are preferably arranged above thefurnace, and each comprises:

-   -   a balance arm,    -   means for measuring a displacement or a force experienced by the        balance arm,    -   an aluminium suspension rod which extends substantially        vertically and has a lower end which is provided with a platinum        hook for attaching a sample and an upper end which is        articulated with or fixed to one longitudinal end of the balance        arm. The latter is called the measuring end of the balance arm;        the other longitudinal end of the balance arm is called the        calibrating end.

As explained above, the balances are advantageously arranged in theshape of a star in order to allow the samples to be placed in a centralchamber of reduced radial dimensions, in which it is simple to create acontrolled homogeneous atmosphere, at reduced cost (use of a smallamount of gas, energy saving for producing the vacuum inside the chamberor the introduction and removal of gas, etc.). The balance arms, thelongitudinal dimensions of which are greater than the radial dimensionsof the chamber, are accordingly preferably arranged in the shape of astar: the balance arm of each balance extends substantially according toa radial direction, for example parallel to the axis of an ellipse ofthe furnace, so that its measuring end hangs over the central portion ofthe furnace and the suspension rod carried by that end therefore extendsin the central portion of the device at the level of a receiving focus(the calibrating end of the balance arm being in the peripheralportion).

The measuring means of at least one balance, and preferably of eachbalance, comprise an electronic weighing cell on which the balance armrests and is fixed. By way of variation, the balance arm rests on afixed blade, on which it is able to oscillate freely, and the measuringmeans comprise optoelectronic means for measuring the displacement of apoint of the balance arm.

The suspension rods are optionally of the capillary type, that is to sayhollow, with two channels permitting the passage of thermocouple wires,such as type S thermocouple wires (platinum/rhodium-plated platinum).

By way of variation, the device comprises means for supporting at leastone piece of material (of the same nature as the material being tested),called the control, which support means are suitable for holding thecontrol in the immediate proximity of a sample, preferably beneath thesample and on a receiving focus, and are equipped with means formeasuring the temperature inside the control. Thesetemperature-measuring means comprise, for example, thermocouple wireswhich end inside the control. Contrary to the preceding embodiment, thethermocouple wires here measure precisely the temperature of thematerial, inside the control, and not the temperature in the vicinity ofa sample. In the presence of the direct heating means according to theinvention, it is not rare to obtain a temperature difference of aboutone (or even several) hundred degrees Celsius between the material andthe atmosphere immediately surrounding it. This latter embodimenttherefore provides a more reliable estimate of the temperature of thesamples being tested (by measuring the temperature of the control) andallows more accurate control of the heating means in order to carry outthe temperature cycles as predetermined. Furthermore, the presence ofthermocouple wires in the suspension rods of the samples can cause notnegligible disturbances in the weight measurements of the samples, andit is advisable to evaluate such disturbances and take them intoaccount. The use of a control arranged beneath and in the vicinity of asample allows this problem to be overcome.

Advantageously and according to the invention, the device comprisessupport means for N controls, which means are suitable for holding acontrol beneath each sample, on a receiving focus, and are equipped withmeans (thermocouple wires ending inside the control) for measuringindependently the temperature of each of the controls. It is thuspossible to regulate the temperature of each sample in an independentmanner, by controlling each lamp in an independent manner.

The furnace is mounted to slide according to a substantially verticaldirection between a bottom preparation position, in which it is locatedbeneath the lower end of the suspension rods in order to allow thesamples to be attached and/or removed, and a top test position, in whichthe lower ends of the suspension rods (and the samples which may beattached thereto) extend inside the chamber of the furnace in order tocarry out a test.

Advantageously and according to the invention, the confining meanscomprise:

-   -   an upper protective bell which is suitable for covering all the        balances and for being fixed in a removable and air-tight manner        to the support plate,    -   a confinement column between the support plate and the furnace,        which column is suitable on the one hand for producing an        air-tight and removable connection, allowing the suspension rods        to pass and be confined, between the support plate and the        chamber of the furnace, and on the other hand for producing an        air-tight and preferably removable connection to means for        generating the controlled gaseous atmosphere. To this end, the        column comprises various branches, especially a branch for its        connection (in an air-tight and preferably removable manner) to        a vacuum pump, a branch for its connection (in an air-tight and        preferably removable manner) to a gas inlet pipe and a branch        for its connection to a safety valve. In addition to the vacuum        pump and the gas inlet pipe for supplying the chamber of the        furnace, the means for generating the controlled gaseous        atmosphere also comprise a gas outlet pipe which opens at an        inner face of the chamber of the furnace, allowing a circulation        of gas to be produced inside said chamber from the gas inlet        pipe to the gas outlet pipe,    -   means for limiting gaseous and thermal exchanges between the        furnace and the weighing means. Advantageously and according to        the invention, the means for limiting gaseous and thermal        exchanges comprise a plurality of superposed and distant plates        which are integrated into the confinement column above the        branches thereof, said plates delimiting a plurality of        successive cooling chambers. Each plate is perforated with N        holes for the passage of the suspension rods; it preferably has        faces of low emissivity. These means advantageously limit on the        one hand thermal exchanges between the furnace and the weighing        means, so that the temperature beneath the protective bell        remains close to ambient temperature (20° C.) and the various        measuring (especially electronic) instruments are preserved. On        the other hand, they limit gaseous currents (especially of        thermal origin) between the furnace and the weighing means,        which currents are liable to generate stresses on the weighing        instruments and influence the measurements.

The protective bell, the support plate, the confinement column and thechamber of the furnace thus form a confined containment having acontrolled atmosphere, inside which the same pressure (but differenttemperatures) prevails. In fact, the means for limiting gaseous andthermal exchanges according to the invention advantageously permitgaseous exchanges of weak flow between the furnace and the weighingmeans, allowing the pressures to be equalised between these two parts ofthe device. An air-tight enclosure for these two parts would have thedamaging consequence of generating pressure differences which mightfalsify the weight measurements of the samples.

Advantageously and according to the invention, each balance alsocomprises a counterweight, preferably made of an inert material (withrespect to the gaseous atmospheres of the different tests to be carriedout), fixed to the calibrating end of the balance arm so as to besuspended inside the protective bell. This counterweight remainspermanently, from one test to another, inside said bell, in a confinedenvironment. At the start of the test, for each sample attached to asuspension rod, there is advantageously added to said rod (which forthat purpose comprises a suitable platinum hook) a tare of an inertmaterial, chosen in order to adjust the balance to zero (which impartsgreater accuracy to the weight measurements). Adjustment of the balanceis thus carried out without having to open the protective bell.Disturbance of the instruments present beneath the bell andcontamination of the atmosphere inside the bell (with dust, undesirablegases, etc.) are thus avoided. The accuracy and reliability of the testare increased thereby, and the time for which the device is immobilisedbetween two tests is reduced considerably.

Advantageously and according to the invention, the furnace comprises atemperature-regulating device of the PID (proportional integralderivative) type or of the predictive and/or self-adaptive type. Such aregulator takes account of past and future evolution to control theheating means in real time.

In addition, the temperature-regulating means of the furnace areadvantageously suitable for controlling each lamp independently,especially as a function of the results of the temperature measurementof the samples or of the controls.

The invention relates also to a thermogravimetric test method anddevice, characterised in combination by all or some of the featuresmentioned hereinabove and hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aims, features and advantages of the invention will becomeapparent upon reading the following description, which refers to theaccompanying figures showing preferred embodiments of the inventionwhich are given solely by way of example and without implying anylimitation and in which:

FIG. 1 is a diagrammatic view in perspective of a thermogravimetric testdevice according to the invention,

FIG. 2 is a diagrammatic vertical section of part of a thermogravimetrictest device according to the invention,

FIG. 3 is a diagrammatic view in perspective of weighing means accordingto the invention,

FIG. 4 is a diagrammatic horizontal section of a furnace according tothe invention,

FIG. 5 is a diagrammatic vertical section of means, according to theinvention, for limiting gaseous and thermal exchanges between thefurnace and the weighing means,

FIG. 6 is a diagrammatic vertical section of part of anotherthermogravimetric test device according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The device shown in FIG. 1 comprises a frame 1 formed of four verticalmembers 2 and various crosspieces and a plate providing rigidity for theframe. At their feet, the vertical members 2 have absorbent means (notshown), which are suitable for absorbing any shocks or vibrationstransmitted to the device by the ground, in order to avoid anydisturbance to the measurements. The frame also comprises a supportplate 3 for the weighing means, to which a plurality of balances arefixed.

The device shown comprises a furnace 4 (see also FIG. 4) which ismounted to slide on two vertical guide rails 5. To that end, the furnacehas two pairs of lateral bearings 17 fixed to its substantiallycylindrical body. The furnace 4 is associated, by way of two bearings 47fixed beneath the support plate 3, with two counterweights 46 allowingon the one hand sliding manoeuvres of the furnace to be facilitated andon the other hand the furnace to be maintained in equilibrium whateverits position along the guide rails 5. The furnace is manoeuvredespecially between a bottom preparation position, allowing access to thesamples 10, and a top test position as shown in FIGS. 1 and 2, in whichit is coupled in an air-tight manner to a confinement column 7.

The furnace 4, shown in FIG. 4, is cylindrical in shape overall and hasan inner face 12 forming six ellipse portions (in any horizontal cuttingplane), the large axes of which extend substantially according toregularly spaced radii of the cylinder in order to define the branchesof a star. In other words, the furnace has rotational symmetry modulo60°, about a longitudinal axis of the furnace (also corresponding to acentral axis of the device).

At the site of the focus 13 of each ellipse (ellipse focus that isfurthest from the centre of the furnace) there is arranged a lamp 11 ofthe halogen lamp type. The focus 13 is called the emitting focus. Eachlamp 11 is connected by sockets 15 and cables 52 to power supply andcontrol means (not shown) comprising PID-type means for regulating thesupply signal, in order to regulate the temperature of the furnace(regulation of the luminous intensity of the lamps—by regulation of theelectric intensity supplied to the lamps—and therefore the temperatureof the samples).

The other ellipse focus, referenced 14, is intended to receive a sampleof material. According to their emission direction, the rays emitted bya lamp 11 are either emitted directly in the direction of the receivingfocus 14 (and of the sample) of the ellipse portion associated with saidlamp, or are reflected by said ellipse portion in the direction of thereceiving focus 14 and of the sample of that ellipse, or are reflectedby another ellipse portion in the direction of another focus, ontoanother sample. In this manner, all the rays emitted by the six lamps 11converge towards the six receiving focuses 14. The site of each focus 14defines, over the entire height of the lamps, a zone of maximumillumination where the rays emitted by the lamps 11 converge.

The six ellipses have separate receiving focuses, so that the furnace isable to accept six samples simultaneously. Each sample is heated by allof the lamps 11 and, in a more negligible manner, by the radiationemitted by the other hot samples. It is possible to adjust thetemperature of a sample 10 in a precise manner by regulating theluminous intensity of a single lamp 11, especially of the lamp situatedon the same ellipse, but this regulation must take into account theluminous intensity of the other lamps at that time.

The furnace also comprises a chamber 9 of optical-grade quartz, whichchamber 9 defines the inner space of the furnace which is intended toreceive the samples and inside which the controlled atmosphere iscreated.

The device also comprises weighing means 6 comprising six independentbalances 38 which are arranged in the shape of a star on the supportplate 3 above the furnace 4, as shown in FIG. 3. The weighing meanstherefore have, inside the furnace, rotational symmetry modulo 60°.

Each balance comprises a balance arm 39 which extends radially above anellipse of the furnace 4 and is fixed to an electronic weighing cell 40integrated into an electronic casing 53. The cell 40 is a cell known perse (it will therefore not be described or shown in detail in the presentpatent) capable of measuring a total weight (balance arm 39, suspensionrod 41, sample 10 and optional counterweight for equilibration of thebalance arm) of less than 80 g with an error of 10 μg, including when itis subjected to a torque (especially in the absence of a counterweight).Such a cell is marketed especially under the trade mark SARTORIUS®.

At its central longitudinal end 45, called the measuring end, thebalance arm 39 carries a suspension rod 41. The rod has a length suchthat, when the furnace is in the test position, locked on theconfinement column 7, its lower end equipped with a sample 10 is locatedat a median height of the furnace 4. At its lower end, the rod 41 has ahook 49 by means of which a sample 10 is attached. The rod 41, which ispreferably made of aluminium, also has two longitudinal channels whichreceive thermocouple wires 48 connected to the PID regulating means ofthe furnace. Said wires pass through the suspension rod to its lowerend, from where they emerge close to the sample 10 in order to measurethe temperature prevailing there.

Each balance also comprises a fixed double stop 50 which allows theangular displacement of the balance arm 39 to be limited in bothdirections, in order to avoid any risk of damage to the weighing cell 40during handling of the device, and especially during attachment of asample to the suspension rod 41 or the removal of a sample.

During operation, the six balances are covered by a single bell 8 forisolating them from the ambient environment. The bell 8 is fixed to thesupport plate 3 by means of a peripheral fixing flange 54. The flangecomprises a lower recess for receiving a seal 42, and a plurality ofbores suitable for each receiving a threaded rod 43 projecting from thesupport plate 3. Each rod 43 has an associated screw in order to keepthe bell firmly flattened against the support plate.

The device also comprises a confinement column 7 in two portions: anupper portion 18, called the insulating portion, having means forlimiting gaseous and thermal exchanges between the furnace and thebalances, and a lower portion 19, called the branching portion, havingbranches for connection to means for generating the controlled gaseousatmosphere. The upper insulating portion 18 is fixed in an air-tightmanner to the support plate 3, by means of a flange 23 screwed to thelower face of said plate and equipped with a seal. The lower branchingportion 19 is fixed, at its upper end, to the lower end of theinsulating portion 18 by means of a clamping flange 21 having conicalbevels, allowing the opposite edges of said portions, between whichthere is interposed a seal 20, to the flattened against one another. Thelower end of the branching portion 19 is fixed in a similar manner, bymeans of clamping flange 26 and a seal, to a fixing collar 24 of thefurnace, when the furnace is in the test position. Following a test, thefixing flange 26 is removed to allow the furnace to slide downwards andto permit access to the samples.

The lower portion 19 comprises a first branch 29 for the connection of agas inlet pipe 28, a second branch 31 for the connection of a pipe 30 ofa vacuum pump (not shown), a third branch opening at a safety valve 32(see FIG. 1), and a fourth branch provided for connecting another device(second gas inlet, measuring device, etc.) if required. Said branchesare produced by any suitable means allowing the apparatus in question tobe connected in an air-tight and optionally removable manner to theconfinement column 7.

The upper insulating portion 18, shown in FIG. 5, comprises acylindrical pipe provided with outer transverse blades 37 for cooling it(these blades limit thermal exchanges by conduction in the wall of thepipe and increase the area of exchange by radiation with the ambientatmosphere), and a series of inner plates 34, the surface area of whichcorresponds to the inside section of the cylindrical pipe and which areseparated by rings 36. The plates 34 and rings 36 are stacked on ashoulder 33 of the pipe. Each plate 34 has six holes 35 for the passageof the suspension rods 41. During the heating steps and at least part ofthe cooling steps of the cyclic test, the gases present in the chamber 9of the furnace have a temperature greater than that of the gases presentbeneath the bell 8. They therefore ascend, by way of the confinementcolumn 7, to the weighing means. The presence of the plates 34 allowsthe phenomena of convection—which are liable to disturb the measurementsof the balances—between the furnace and the containment in which thebalances are arranged, to be limited, while allowing gaseous exchangesof weak flow between said furnace and said containment in order toobtain a substantially identical pressure between these two parts of thedevice (a pressure difference would act on the balances and falsify themeasurements). It also allows the gases coming from the furnace andescaping towards the weighing means to be cooled: two successive platesin effect form a cooling chamber 55 into which the gases coming from thefurnace 4 enter through the holes 35 and expand. The plates 34preferably have faces of low emissivity in order also to reduce thermalexchanges by radiation between two consecutive plates. The use of aplurality of such plates allows the atmosphere prevailing beneath thebell 8 to be insulated thermally, in an effective manner and with areduced space requirement, from the atmosphere of the furnace 4. Thedevice also comprises, optionally, a plug 51 pierced with six holes forthe passage of the rods 41, which plug 51 seals off a central hole ofthe support plate 3.

According to the invention, the device shown is used as follows:

-   -   the furnace is placed in the bottom position in order to allow        samples 10 to be attached to the suspension rods 41; it is to be        noted that it is possible to attach a plurality of samples to        the same rod within the limits of the capacities of the balance,        but this is not desirable in so far as the measuring accuracy        obtained is generally less good,    -   the furnace is slid into its test position, and the furnace is        coupled to the column 7 in an air-tight manner by means of the        clamping flange 26; the chamber 9 of the furnace, the        confinement column 7, the support plate 3 and the bell 8 then        form a confined containment with a controlled atmosphere,    -   an atmosphere is generated within the containment: depending on        the material being tested, the vacuum is produced inside the        containment by means of the vacuum pump and/or a gas, for        example a corrosive gas, is introduced into the containment via        the gas inlet pipe 28; the introduction can be carried out        bubble by bubble if a very low pressure (primary or secondary        vacuum) is desired inside the containment (and has been produced        by means of the vacuum pump); the gas introduced is evacuated        via the gas outlet pipe 27 in the lower portion of the chamber 9        of the furnace;    -   the samples are subjected to predetermined thermal cycles as        defined above: the electric intensity delivered to the lamps 11        is adjusted in real time by the PID regulating means as a        function of the programmed thermal cycles and the actual        temperature of the samples as measured by the thermocouples 48;        the regulation can be carried out on each lamp independently or        on several lamps jointly; the atmosphere generated is monitored        at all times throughout the test, and it is possible to alter        the atmosphere generated (in terms of pressure and/or chemical        composition) during the test, and especially from one cycle to        another or during the same cycle, or even during each cycle;    -   during each thermal cycle, the weight of each sample 10 as        measured by the associated balance 38 is recorded continuously        (in a memory of the computer means for controlling said        balances, not shown) at least during the high-temperature stage,        or even continuously throughout the test.

Following the test (for example after 3000 consecutive cycles), thefurnace is detached from the confinement column 7 and is displaced to apreparation position in order to remove the samples.

The recorded weight measurements are transmitted to processing computermeans (which may or may not be an integral part of the device) so thatthey can be presented, especially in the form of graphs (such as a graphshowing the variation in mass Δm of a sample as a function of time),and/or analysed and/or applied to simulation software means.

FIG. 6 shows a variant of the weighing means and of the means formonitoring and regulating the furnace. Each balance 6 comprises apermanent counterweight 56 attached to the longitudinal end 44 of thebalance arm, opposite its measuring end 45 and called the calibratingend, inside the protective bell 8. The counterweight is chosen to be ofan inert material and to have a weight slightly greater than the typicalweight of a sample.

Before the start of a test, the balance arm is equilibrated so as toadjust the balance to zero, by means of a tare 59 attached to thesuspension rod 41. This operation results in better reliability of theweight measurements, the balance not being subjected to torque whichmight prejudice the accuracy of the measurements. The tare 59 is chosento be of an inert material in order to have a constant weight throughoutthe test. The hook for receiving the tare is located beneath thecontainment column 7 so as to be accessible when the furnace is in thebottom preparation position.

In this manner it is not necessary to remove the protective bell 8 or toopen the confinement column 7 in order to carry out equilibration of thebalance before a test. The volume containing the weight-measuringinstruments therefore remains confined between two tests. Thisconfinement is not completely air-tight because exchanges of gas of weakflow with the outside can occur by way of the holes 35 for the passageof the suspension rods 41. However, it is sufficient to protect themeasuring instruments 39, 40, 53, etc. from dust and from any thermalshock or any sudden variation in pressure, which might damage them. Thisadds to the durability of these instruments, which are particularlysensitive and fragile, and ensures that the weight measurements remainreliable test after test.

In addition, the device shown in FIG. 6 has six rigid rods 60 forsupporting six controls 57. The controls 57 are pieces made of the samematerial as the samples to be tested or of a material having identicalthermal properties (capacity, conductivity, absorptivity, emissivity,etc., it being necessary for the samples and the controls to havesubstantially the same temperature if they are subjected to the sameradiation energy).

Each support rod 60 extends plumb with a suspension rod 41 in order toallow a control 57 to be placed below and in the immediate proximity ofa sample 10, on a receiving focus 14 of the furnace. The control istherefore subjected strictly to the same radiation as the sampledirectly above it. The temperatures of the control and of the sample aretherefore identical.

Each support rod 60 is hollow, so that it can receive thermocouple wires58, which pass through the bottom of the furnace in an air-tight mannerand are connected to the PID regulating means of the furnace. Thesuspension rods 41 for the samples therefore do not have thermocouplewires, which again contributes to greater accuracy of the weightmeasurements.

The measuring end of each thermocouple wire 58 is embedded inside thecontrol 57 in a corresponding recess provided for that purpose, so thatthe temperature detected corresponds exactly to the temperature of thematerial (of the control and of the sample) and not to the temperatureof the atmosphere close to a sample. It is possible in this manner tocontrol the heating means accurately in order to carry out theprogrammed cycles.

Many variations of the invention compared with the embodiments shown anddescribed are possible.

In particular, the number N of samples tested simultaneously is notlimited to six (as shown). The number is, however, dictated by thedesired use of the device (study, research, industrial validation tests,statistical studies, etc.), the maximum space available foraccommodating the device, and the space available for the balances used.Owing to its overall star-shaped structure, the device according to theinvention as shown is particularly compact.

In addition, the type of balance used is not limited to that shown(balance having an electronic weighing cell, balance with optoelectronicor magnetic means for measuring the displacement of the balance arm,etc.) and one device may incorporate balances of different types.

The required accuracy for each balance depends on the nature of thematerial being tested and the environmental and thermal conditions ofthe test. An error of less than 100 μg allows a large majority of teststo be carried out; an error of less than 10 μg is suitable for the mostdifficult tests (such as tests of oxidation of superalloys for theaviation industry).

Moreover, the means for measuring the temperature of the furnace and ofthe samples are not limited to those shown. The device may comprise acentral pipe for the passage of thermocouple wires extending between thesupport plate of the balances and the chamber of the furnace, in themiddle of the suspension rods, and passing through the insulating plates34. By way of variation, temperature sensors equipped with wireless datatransmission means are provided in the chamber of the furnace, in theimmediate proximity of the samples. By way of variation, one of thesamples is used as temperature control: it is attached to a suspensionrod (such as 41) through which thermocouple wires pass (the othersuspension rods being without thermocouples); and the measurements ofthe weight or variations in weight of that sample are not taken intoaccount in subsequent studies. By way of variation, the only rodincorporating thermocouples is left free (it does not carry a sample).

1. A method for thermogravimetrically testing the behaviour of a solidmaterial in the presence of a controlled gaseous atmosphere, wherein: aplurality of samples (10) are placed in the presence of said gaseousatmosphere inside the same controlled-atmosphere furnace (4), eachsample has its own associated balance (38) having an error of less than100 μg, the samples (10) are subjected to successive predeterminedthermal cycles each comprising a heating step, during which the samplesare heated directly, and a cooling step, during which the samples arenot heated, the weight of each sample is measured and recordedindependently, in a continuous manner, at least during ahigh-temperature stage of the heating step of each thermal cycle.
 2. Themethod as claimed in claim 1, wherein, in each thermal cycle, thesamples (10) are heated so that their temperature is from 400° C. to1800° C. at least during a high-temperature stage of the heating step.3. The method as claimed in claim 1, wherein, in each thermal cycle, thesamples (10) are heated so that their temperature is greater than 1100°C. at least during a high-temperature stage of the heating step.
 4. Themethod as claimed in claim 1, wherein, in each thermal cycle, thesamples (10) are heated at a rate of heating greater than 300° C.minute.
 5. The method as claimed in claim 1, wherein, in each thermalcycle, the samples (10) are cooled at a rate of cooling greater than100° C. minute.
 6. The method as claimed in claim 1, wherein the samples(10) are subjected to thermal cycles each comprising a heating step,which consists of a phase of rise in temperature having a duration ofless than 5 minutes and a high-temperature stage having a duration ofthe order of 60minutes, and a cooling step, which consists of a phase offall in temperature having a duration of less than 10minutes and alow-temperature stage having a duration of from 0 to 15 minutes.
 7. Themethod as claimed in claim 1, wherein the samples (10) are subjected toa number of successive thermal cycles of from 10 to
 3000. 8. A devicefor thermogravimetrically testing the behaviour of a solid material inthe presence of a controlled gaseous atmosphere, comprising: a furnace(4) having a controlled gaseous atmosphere, means (6) for weighing thematerial placed in the furnace, having an error of less than 100 μg,confining means (7, 8, 34) suitable for limiting any disturbance to theweighing means owing to the external environment of the device and/orthe controlled gaseous atmosphere, wherein the furnace (4) is suitablefor receiving a number N, which is strictly greater than 1, of samples(10) of the material, the furnace comprises means (11) for heating thesamples directly, which means are capable of subjecting the samples tosuccessive predetermined thermal cycles each comprising a heating step,during which the samples are heated, and a cooling step, during whichthe samples are not heated, the heating means being capable of imposinghigh-temperature stages during the heating steps, the weighing meanscomprise N independent balances (38) having an error of less than 100μg, each balance being capable of measuring and recording the weight ofa sample continuously at least during a high-temperature stage of theheating step of each thermal cycle, the device has a star-shapedstructure overall, in which at least the balances are arranged in theshape of a star, which star-shaped structure is suitable for receivingthe samples close to one another in a central portion of the furnace. 9.The device as claimed in claim 8, wherein the direct heating means (11)are capable of bringing the samples to a temperature greater than 1100°C.
 10. The device as claimed in claim 8, wherein the direct heatingmeans (11) are capable of heating the samples at a rate of heatinggreater than 300° C. minute.
 11. The device as claimed in claims 8,wherein the direct heating means (11) are capable of cooling the samplesat a rate of cooling greater than 100° C. minute.
 12. The device asclaimed in claim 8, wherein the direct heating means (11) are capable ofcarrying out thermal cycles each comprising a heating step, whichconsists of a phase of rise in temperature having a duration of lessthan 5 minutes and a high-temperature stage having a duration of theorder of 60 minutes, and a cooling step, which consists of a phase offall in temperature having a duration of less than 10 minutes and alow-temperature stage having a duration of from 0 to 15 minutes.
 13. Thedevice as claimed in claim 8, wherein the direct heating means (11) arecapable of carrying out more than 3000 successive thermal cycles. 14.The device as claimed in claim 8, wherein each balance (38) has an errorof less than 10 μg.
 15. The device as claimed in claim 8, wherein eachbalance (38) has a drift of less than 1 μg/h.
 16. The device as claimedin claim 8, wherein the balances (38) are mounted on the same supportplate (3).
 17. The device as claimed in claim 8, which comprises means(60) for supporting at least one piece of material (57), called acontrol, which means are suitable for holding the control in theimmediate proximity of a sample (10) and are equipped with means (58)for measuring the temperature inside the control.
 18. The device asclaimed in claim 17, which comprises means (60) for supporting Ncontrols, which means are suitable for holding a control beneath eachsample, on its receiving focus, and are equipped with means (58) of thethermocouple wire type which end inside the control, for measuringindependently the temperature of each of the controls.
 19. The device asclaimed in claim 8, wherein the furnace (4) is mounted to slideaccording to a substantially vertical direction between a bottompreparation position, in which it is located beneath the lower end ofthe suspension rods (41) in order to allow samples to be attached and/orremoved, and a top test position, in which the lower end of thesuspension rods (41) extends inside the chamber (9) of the furnace. 20.The device as claimed in claim 8, wherein the confining means comprisean upper protective bell (8) which is suitable for covering all of thebalances (38) and for being fixed in a removable and air-tight manner tothe support plate (3).
 21. The device as claimed in claim 8, wherein theconfining means (7) comprise a confinement column between the supportplate (3) and the furnace (4), which column is suitable for producing,on the one hand, an air-tight and removable connection, allowing thesuspension rods to pass and be confined, between the support plate andthe chamber of the furnace, and, on the other hand, an air-tightconnection, by means of branches (29, 31), to means for generating thecontrolled gaseous atmosphere.
 22. The device as claimed in claim 8,wherein the means for generating the controlled gaseous atmospherecomprise, on the one hand, a vacuum pump and a gas inlet pipe (28) whichare each connected to a branch (29) of the confinement column, and, onthe other hand, a gas outlet pipe (27) which opens at a lower face ofthe chamber (9) of the furnace.
 23. The device as claimed in claim 8,wherein the confining means comprise means (34, 36, 37) for limitinggaseous and thermal exchanges between the furnace and the weighingmeans, said limiting means comprising a plurality of superposed anddistant plates (34) which are integrated into the confinement column (7)above the branches thereof and which delimit a plurality of successivecooling chambers (55), each plate being pierced with N holes (35) forthe passage of the suspension rods.
 24. The device as claimed in claim23, wherein each plate (34) has faces of low emissivity.
 25. The deviceas claimed in claim 8, wherein the furnace (4) comprisestemperature-regulating means of the PID type.
 26. The device as claimedin claim 8, wherein the furnace comprises temperature-regulating meanssuitable for controlling each lamp (11) independently.
 27. The device asclaimed in claim 8, wherein the furnace comprises at least Nhigh-radiation lamps (11), a chamber (9) for receiving the samples, madeof a thermal resistant material that is transparent to the radiation ofthe lamps, and a reflective peripheral inner face (12) having a shapethat is suitable for defining at least N separate zones of maximumillumination inside the chamber, at the site of which the samples may beplaced.
 28. The device as claimed in claim 27, wherein the peripheralinner face (12) of the furnace forms at least,a N ellipse portionsarranged in a star, each ellipse having a first focus (13) outside thechamber (9), called the emitting focus, at the site of which there isarranged a lamp, and a second focus (14) inside the chamber, called thereceiving focus, at the site of which a sample may be placed, at least Nof said ellipses having separate receiving focuses.
 29. The device asclaimed in claim 28, wherein the chamber (9) and the receiving focuses(14) are situated in the central portion of the furnace and the emittingfocuses (13) are situated in the peripheral portion of the furnace, andwherein the chamber (9) has reduced radial dimensions.
 30. The device asclaimed in claim 8, wherein the balances (38) are arranged above thefurnace and each comprise a balance arm (39), means (40) for measuring adisplacement or a force to which the balance arm is subjected, and asuspension rod (41) of aluminium which extends substantially verticallyand has a lower end provided with a hook (49) for the attachment of asample (10) and an upper end articulated with or fixed to a longitudinalend (45) of the balance arm, called the measuring end.
 31. The device asclaimed in claim 30, wherein the balance arms (39) of the N balances arearranged in the shape of a star, each balance arm extendingsubstantially according to a radial direction so that its measuring end(45) hangs over the central portion of the furnace.
 32. The device asclaimed in claim 30, wherein the suspension rods (41) are of thecapillary type having two channels in order to permit the passage ofthermocouple wires (48).
 33. The device as claimed in claim 30, whereinthe measuring means of at least one balance comprise an electronicweighing cell (40) to which the balance arm (39) is fixed.
 34. Thedevice as claimed in claim 30, wherein each balance comprises apermanent counterweight (56) fixed to one longitudinal end (44) of thebalance arm, called the calibrating end, so as to be suspended insidethe protective bell (8).